Laminated composite materials, in which reinforcing fibres are held within a polymeric matrix, are extensively used in many engineering applications. Such materials can generally provide a higher strength and stiffness per unit weight than conventional metallic materials. This makes such composite materials advantageous for weight sensitive applications, such as those in the field of aerospace.
A known problem with laminated composite materials is their poor inter-laminar, or through-thickness, mechanical properties in comparison to the corresponding in-plane properties. Such low inter-laminar strength and fracture toughness can constrain the design of composite parts and may even limit the use of such materials for certain applications.
One solution to this problem is the use of a toughened matrix material. Such matrix materials are generally significantly more expensive than conventional matrix materials, often have poor high temperature properties and may still not provide a sufficient increase in fracture toughness.
An alternative solution to improving inter-laminar strength properties is the insertion of through-thickness fibres into the laminated material. Various techniques have been developed for the insertion of such reinforcing fibres.
One such technique involves the use of stitching to insert the through-thickness reinforcement fibres. This requires the use of a needle which can be in excess of 5 mm in diameter. When penetrating a fibre laminate with a needle of this size, significant cutting and deformation of the laminate's fibres can be caused. This may result in reductions in the in-plane material properties in excess of 20%.
In addition, the use of a continuous fibre for the through-thickness reinforcement may cause a kink to form in the top few plies of the laminate as a result of the loop of fibre traversing from one stitch to the next. This can also result in a reduction in the in-plane properties of the material.
An alternative method for inserting through-thickness reinforcing fibres into the laminated material is stapling or z-pinning. These reinforcing fibres are generally fibrous in structure and formed with a 45° chamfer at the insertion end.
A problem with conventional z-pinning techniques is that of the reinforcing fibres being deflected from the desired insertion trajectory by the interaction of the end chamfer with the densely packed fibres of the laminate. This can result in the inserted fibres being incorrectly angled or curved relative to the plane of the laminate, which can significantly reduce their through-thickness reinforcing properties.
A further problem with the conventional z-pinning process is that the fibrous reinforcing pins can split or fracture during insertion as a result of the need to force the pins through the densely compacted fibre layers of the laminate. This can reduce the effectiveness of the presence of the reinforcing pins.